Pinner school sinkhole: 39km drilling stabilised chalk mines
Engineers injected over 5,500 tonnes of grout to secure medieval mine network beneath Pinner Wood School.
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A significant ground stabilisation operation was completed beneath Pinner Wood School after a sinkhole in its car park in March 2017 revealed extensive medieval chalk mines. The work involved more than 39 kilometres of drilling and the injection of over 5,500 tonnes of grout to secure the subterranean network.
The sinkhole incident at the school, located in Pinner, Harrow, led to its temporary closure. Experts identified evidence of collapsing passages beneath the school grounds, raising concerns about the stability of the building and playground. Following the nine-month grouting and ground investigation, pupils were able to return to the site.
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Ancient mining history
The mines beneath Pinner are part of a network dating back to at least 1388, when court rolls recorded 'marlpits' in the vicinity. Chalk extraction was historically carried out for agricultural use, with flint also being a byproduct. The church on Pinner High Street is believed to have been constructed using chalk from these mines.
Records from 1652 show John Finch was fined £5 for illegally extracting 100 loads of chalk from land belonging to the lord of the manor. Evidence of miners' names and dates, such as J. Gumm who worked between 1855 and 1870, have been found scorched into the chalk walls.
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The use of chalk for building materials increased from the 16th century onwards, and by the 18th century, local construction firms were utilising it. Miners typically worked in pairs, excavating chalk and hoisting it to the surface via vertical shafts.
Previous ground collapses
The risks associated with the mines have been evident from previous ground collapses. In 1954, a collapse occurred on Uxbridge Road, believed to have been exacerbated by a bomb detonation in 1941 and subsequent modern traffic. Substantial concrete was pumped into the mines to stabilise the area.
Further sinkholes, approximately six metres deep, appeared near the entrance to Montesole Allotments in 1983 following severe water ingress. Despite these incidents, buildings have been permitted above some former mine workings, including a block of flats at The Dingles and houses on Albury Drive.
Access sealed
Harrow Council has since sealed access to the Pinner chalk mines for health and safety reasons. Despite the stabilising efforts, the existence of at least three known entrances and numerous tunnels means further collapses and sinkholes cannot be entirely ruled out, as reported by MyLondon.
Questions this report answers
+What caused the sinkhole at Pinner Wood School?
A sinkhole opened in the car park of Pinner Wood School in March 2017. This revealed an extensive network of medieval chalk mines beneath the school grounds, leading to fears the building and playground could collapse.
+What work was done to stabilise the mines?
Following the sinkhole, engineers undertook a nine-month operation. This involved over 39 kilometres of drilling and the injection of more than 5,500 tonnes of grout to stabilise the chalk mine tunnels under the school.
+How old are the chalk mines in Pinner?
The chalk mines beneath Pinner date back to at least 1388, when 'marlpits' were recorded in local court rolls. Mining activity continued for centuries, with chalk used for agriculture and building materials.
+Is access to the mines still open?
No, Harrow Council has sealed access to the Pinner chalk mines for health and safety reasons. Despite this, the extensive nature of the tunnels means further ground instability cannot be entirely ruled out.
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